This paper presents power-control strategies of a grid-connected hybrid generation system with versatile power transfer. The hybrid system is the combination of photovoltaic (PV) array, wind turbine, and battery storage via a common dc bus. Versatile power transfer was defined as multimodes of operation, including normal operation without use of battery, power dispatching, and power averaging, which enables grid- or user-friendly operation. A supervisory control regulates power generation of the individual components so as to enable the hybrid system to operate in the proposed modes of operation. The concept and principle of the hybrid system and its control were described. A simple technique using a low-pass filter was introduced for power averaging. A modified hysteresis-control strategy was applied in the battery converter. Modeling and simulations were based on an electromagnetic-transient-analysis program. A 30-kW hybrid inverter and its control system were developed. The simulation and experimental results were presented to evaluate the dynamic performance of the hybrid system under the proposed modes of operation.

Dynamic Modeling and Control of a Grid-Connected Hybrid Generation System With Versatile Power Transfer

This paper deals with nonunit protection of HVDC grids by proposing a set of parameters that characterizes the open protection zones together with an efficient method to determine the thresholds on these parameters. Selective HVDC grid protection schemes must detect and discriminate faults within the first milliseconds of the fault transient and consequently differ considerably from existing ac protection schemes. Due to the accompanying speed requirement, primary protection is expected to be based on open protection zones as communication delay impedes fast operation. In this paper, the principles of the nonunit protection scheme are developed based on reflection of a traveling wave at an inductive termination. Next, the method to obtain the protection scheme thresholds is elaborated. The method accurately calculates the thresholds for HVDC grids with an arbitrary topology. A sensitivity analysis of these thresholds toward grid and fault parameters demonstrates the applicability of the proposed protection scheme in cable-based HVDC grids with inductive cable termination. The results obtained with the reduced grid model are validated by comparison against simulations using a detailed model implemented in PSCAD.

Nonunit Protection of HVDC Grids With Inductive DC Cable Termination

This paper presents a new solution method that combines state-space and nodal analysis for the simulation of electrical systems. The presented flexible clustering of state-space-described electrical subsystems into a nodal method offers several advantages for the efficient solution of switched networks, nonlinear functions, and for interfacing with nodal model equations. This paper extends the concept of discrete companion branch equivalent of the nodal approach to state-space described systems and enables natural coupling between them. The presented solution method is simultaneous and enables benefitting from the advantages of two different modeling approaches normally exclusive from one another.

A Combined State-Space Nodal Method for the Simulation of Power System Transients

This paper presents the hardware-in-the-loop (HIL) simulation of power electronic systems using a unique adaptive discretization technique based on the input of the system, where the coefficient matrices and system equations are changed while the simulation is running. The voltage-source-converter (VSC)-based HVDC system is used as a case study. Two z-transform-based discretization techniques, known as the step-invariant transformation (SIT) and the ramp-invariant transformation, and one of their derivative time-shifted SIT are applied for simulating the VSC-HVDC system. Discrete switching synchronization algorithms are used to accommodate the asynchronous events that take place in between two discrete simulation points. The HIL simulation is implemented using an off-the-shelf PC cluster interfaced with a digital controller. The simulator and the controller are connected through I/O ports, which facilitate the exchange of analog and digital signals. A 4-kW VSC-HVDC experimental setup is used to validate the simulation results, using the same digital controller as that in the HIL simulation to supply the necessary gate pulses for the experimental VSCs. A comparative study of the results obtained through offline, HIL simulation, and the experiment is presented.

Hardware-in-the-Loop Simulation of Power Electronic Systems Using Adaptive Discretization

A survey on intelligent system application to fault diagnosis in electric power system transmission lines

Electromagnetic transients simulation (EMTS) has become a universal tool for the analysis of power system electromagnetic transients in the range of nanoseconds to seconds. This book provides a thorough review of EMTS and many simple examples are included to clarify difficult concepts. This book will be of particular value to advanced engineering students and practising power systems engineers.

Power Systems Electromagnetic Transients Simulation

In this chapter, the authors discusses the travelling wave transmission line models. The frequency dependent transmission line model was presented. Details of transmission line geometry and conductor data are required in order to calculate accurately the frequency-dependent electrical parameters of the line. The simulation time step was based on the shortest response time of the line. The chapter also presents the phase-domain models which is the most accurate and robust for detailed transmission line representation. Calculation of electrical parameters for overhead power transmission lines and underground power cables were also demonstrated.

Transmission lines and cables

A continuous function can be simulated by substituting a numerical integration formula into the differential equation and rearranging the function into an appropriate form. Among the factors to be taken into account in the selection of the numerical integrator are the error due to truncated terms, its properties as a differentiator, error propagation and frequency response. Numerical integration substitution (NIS) constitutes the basis of Dommel's EMTP , which, as explained in the introductory chapter, is now the most generally accepted method for the solution of electromagnetic transients.

Numerical integrator substitution

Frequency dependent network equivalents are important for modelling modern power systems due to their size and complexity. The first stage is to determine the response of the portion of the network to be replaced by an equivalent, as seen from its boundary busbar(s). This is most efficiently performed using frequency domain techniques to perform a frequency scan. Once determined, a rational function which is easily implemented can be fitted to match this response.

Frequency dependent network equivalents

With the exceptions of a few auxiliary components, the electrical power system is a continuous system, which can be represented mathematically by a system of differential and algebraic equations. A convenient form of these equations is the state variable formulation, in which a system of n first-order linear differential equations results from an n order system. The state variable formulation is not unique and depends on the choice of state variables. The following state variable realisations have been described in this chapter: successive differentiation, controller canonical, observer canonical and diagonal canonical. Digital simulation is by nature a discrete time process and can only provide solutions for the differential and algebraic equations at discrete points in time, hence this requires the formulation of discrete systems. The discrete representation can always be expressed as a difference equation, where the output at a new time point is calculated from the output at previous time points and the inputs at the present and previous time points.

Jos Arrillaga

Papers

Power Systems Electromagnetic Transients Simulation

Transmission lines and cables

Numerical integrator substitution

Frequency dependent network equivalents

Analysis of continuous and discrete systems